1. Field of the Invention
This invention relates to mechanisms for converting large mill rolls of synthetic resin or plastic films to product rolls of the film after it has been perforated by forming a plurality of perforations or score lines extending transversely across the film and spaced along its length.
2. Brief Description of the Prior Art
Various types of machines have heretofore been provided for the purpose of unwinding elongated strips or bands of synthetic resin films from large rolls of this material in which the film is stored after being produced by the manufacturer. The film thus unwound is generally placed in a more marketable condition for acquisition by the consumer or ultimate user, and this so-called conversion of the film is frequently in the form of winding the film upon product rolls and slitting, perforating or performing some other operation upon the film as it passes between the large mill roll and the product roll. One type of machine which has been used to perform these functions has a cutting bar which carries a plurality of serrated teeth, and which scores or perforates the film at spaced intervals along its length so as to provide a series of interconnected sheets which can be severed in increments from the film by tearing along the score or perforation lines as the film is unwound by the user from the product roll. The cutter bar thus provided has required relatively frequent replacement, and is difficult and time consuming to install. Moreover, it is susceptible to undesirable misalignment over its length so that the perforating function carried out by this structural element is not uniform across the transverse dimension of the film which is perforated.
The machines previously in use for unwinding mill rolls and producing perforations at spaced intervals along the length of the film have generally been manually fed machines in which it is necessary to manually locate and position a new mill roll at the unwinding position after a preceding mill roll has been unwound and has been removed from the machine. This manual placement is difficult because the mill rolls are frequently very heavy and bulky, and often must be elevated to a relatively high position within the machine in order to provide a readiness status for unrolling.
Difficulties have also been encountered with many of the types of converting apparatus previously used for unwinding mill rolls and perforating the film, in that the wind which is performed in producing the product roll is not uniform, and there is considerable wastage entailed as a result of wrinkling or failure to achieve a smooth, even transfer to the product roll. A portion of this difficulty has resulted from attempts to achieve a uniform braking effect on the unwinding mill roll by braking a central shaft upon which the mill roll is mounted, and with which it moves in rotation. Attempts to achieve braking of the mill roll in this fashion have been ineffective to achieve constant and uniform braking over the unwinding period since the inertial force of the mill roll varies as its diameter changes during the unwinding operation, and an attempt to brake the roll by drag forces applied to the rotating shaft upon which it is mounted has caused a variation in the rate of unwinding, and consequently in the rate in which the film is taken up on the product roll.
Another problem which has been frequently encountered in converter machines of the type under discussion is that of obtaining an accurate count of the number of sheets in the product roll as such sheets are defined by adjacent perforations formed transversely in the film during its course of travel from the mill roll to the product roll. The inaccuracy in the counting or identification of the number of sheets in a product roll has sometimes arisen from the attempt to make such count by counting the number of revolutions of the product roll as the film is taken up thereon. As has been pointed out, the product roll does not turn at uniform speeds at all times during the winding up of the film thereon, nor is the film transferred at a constant speed during the entire period of transfer from the mill roll to the product roll. Also, the size of each succeeding convolution of rolled up film is larger than the last preceding convolution. These factors induce error in any attempt to accurately count the number of sheets formed in the film by counting revolutions of the shaft upon which the product roll is mounted. Therefore, an indication of the number of sheets contained in a product roll is generally taken only as an approximation, with a margin of error always being recognized.
More often, the problem of sheet count arises when a first count is made on the basis of lineal feet of film going into the product roll, and then attempting to mathematically convert this figure to the number of sheets in the product roll.
Situations are frequently encountered in the conversion of plastic film from mill roll to product roll of desiring to run, simultaneously, a pair of relatively short length mill rolls, and to take up the film from these two mill rolls on two separate product rolls. This has been attempted with several machines by mounting the mill rolls for concurrent rotation on a common shaft to which the mill rolls are keyed, and with which they therefore rotate. The inherent variation in the speeds with which the film will be taken off the two mill rolls and transferred to the two product rolls results in an uneven distribution of the film on the two product rolls, some wrinkling and some wastage of film. To my knowledge, no attempt has been made to the present time to provide for independent rotation of the two concurrently unwound mill rolls so that the independent and differing unwinding forces transferred to each of these rolls in the course of unwinding is automatically compensated by the ability of each roll to move at an independent rate of rotation.